System for controlling driving torque of vehicle

ABSTRACT

In order to provide an accurate traction control by closing a second throttle valve provided in series to an accelerator-operated ordinary throttle valve, with a simplified actuator, a control system for reducing a vehicle driving torque employs one or more sensors for sensing a second vehicle operating parameter representing a road surface friction coefficient (μ) or a driver&#39;s command for acceleration, in addition to sensors for sensing a first vehicle operating parameter representing a drive wheel slip. When the road surface friction coefficient is high or when the driver depresses the accelerator pedal hard, the control system restrains the closing operation of the second throttle valve in accordance with the second parameter even though the closure of the second throttle valve is requested by the first parameter.

BACKGROUND OF THE INVENTION

The present invention relates to a vehicle driving torque control systemor traction control system, and more specifically to a control systemarranged to restrain a drive wheel slip by closing a second throttlevalve provided in series to an ordinary accelerator-operated throttlevalve in an intake passage of an engine.

On a slippery road such as a wet asphalt road or a snow-clad road, thedrive wheels tend to slip during acceleration, and this drive wheelslippage degrades the accelerating performance of the vehicle and thestability of the vehicle more or less.

Therefore, there has been proposed a driving torque control systemintended to improve the starting and accelerating ability of the vehicleand to improve the stability of the vehicle by preventing a rear endswing of the vehicle, by using a second throttle valve disposed in theintake passage of the engine, in series to an ordinary throttle valveconnected with the accelerator pedal. In this conventional system, a DCmotor (stepper motor) is used to actuate the second throttle valve. Whenthe system detects a slip by monitoring a rotational speed differencebetween a drive wheel speed and a non-drive wheel speed or some othervehicle operating condition, then the second throttle valve is closed todecrease the driving force (engine torque).

The DC motor can control the opening degree of the second throttle valveprecisely. However, the DC motor can become a factor increasing the costspecifically when the control system is aimed to improve the startingand acceleration performance only in a low vehicle speed range for a FFvehicle. Moreover, the DC motor with a speed change mechanism is heavyin weight. In general, the DC motor is integral with a throttle chamber,so that the versatility is poor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a vehicle drivingtorque control system which enables employment of a low-cost, two-stepcontrol arrangement for controlling the second throttle valve between afully open position and a predetermined closed position, and yetprovides control performance suitable to the changing vehicle operatingsituation.

Preferably, the control system according to the present inventionemploys a simplified actuator of a two-step (or on-off) control type forcontrolling the second throttle valve alternatively between the open andclosed positions, rather than the DC motor of the conventional elaboratesystem.

In this case, if the opening degree of the second throttle valve in theclosed position is set too low, the control system reduces the vehicledriving force excessively in response to a slip on a road surface havinga relatively high friction coefficient, so that, after escape from theslip condition, the driver cannot accelerate the vehicle responsively.If, on the other hand, the opening degree in the closed position is settoo high, the control system cannot reduce the driving torquesufficiently on a snowy road, in particular a road covered with pressedand hardened snow or icy road (as shown in FIG. 12).

Therefore, in the simple on-off control system alternating between thefully opened position and the closed position, it is difficult to ensurethe adequate performance for various road surface conditions ofdifferent friction coefficients.

Moreover, when the driver's demand for accelerating the vehicle isstrong to escape from a stuck condition, for instance, the closure ofthe second throttle valve results in an unwanted sharp decrease of theengine output torque.

Therefore, a more concrete object of the present invention is to providea vehicle driving torque control system which can properly fulfill avariety of requirements on various road surface conditions by combiningthe simple cost-effective on-off control system regulating the secondthrottle valve, and another driving torque control system.

Another object of the invention is to provide a vehicle driving torquecontrol system which can properly respond to a driver's intention ofacceleration

According to one aspect of the present invention, as shown in FIG. 1, andriving torque control system for a vehicle, comprises: (i) a secondthrottle valve 104 which is disposed in series to a first throttle valvein an intake passage for an engine; (ii) a slip condition sensing means100 for sensing a drive wheel slip condition of the vehicle; (iii) aroad condition sensing means 101 for sensing a frictional condition of aroad surface under the vehicle; (iv) a first vehicle driving torquereducing means 102 for reducing a vehicle driving torque of the vehicleby closing said second throttle valve in accordance with the drive wheelslip condition sensed by said slip condition sensing means 100; (v) asecond vehicle driving torque reducing means 103 for reducing thevehicle driving torque in accordance with the drive wheel slip conditionin a more responsive manner than said first reducing means 102; and (vi)a torque reduction limiting means 105 for limiting an operation of saidfirst driving torque reducing means 102 in a road surface condition of ahigh friction coefficient.

When the slip condition sensing means 100 detects an occurrence of drivewheel slip, the first torque reducing means 102 closes the secondthrottle valve 104 to a predetermined closed position in accordance withthe degree of the drive wheel slip, and thereby restrains the slip byreducing the vehicle driving torque.

When, however, the friction coefficient of the road surface on which thevehicle is running is high, the torque reduction limiting means 105 canlimit the control operation of the first torque reducing means 102, andleave the second torque reducing means 103 in control operation torestrain the slip. On a road surface having a high friction coefficient(μ), reacceleration after suppression of the slip requires a significantamount of driving force. In other words, on a high μ road surface wherea slip is less likely to occur, the vehicle driving force immediatelybefore an occurrence of slip is at a high level, and accordingly a largeengine output is required for restoration to that high level.

If the second throttle valve 104 is closed in such a high μ road surfacecondition by the first torque reducing means 102, the engine will beslow to increase its output torque after recovery from slip because of adelay from a reopening operation of the second throttle valve 104 aftersuppression of slip until an actual increase of the intake air flowrate, and accordingly the accelerating performance will be poor. Thelimiting means 105 can avoid this problem by acting to hold the secondthrottle valve 104 open on such a less slippery road surface and toleave the slip control operation to the second torque reducing means103. The control system shown in FIG. 1 can provide a superioraccelerating performance by increasing the driving torque quickly bycanceling the slip control operation of the second torque reducing means103.

The first torque reducing means 102 is inferior in responsecharacteristic or response speed to the second torque reducing means103. However, the throttling of the intake air flow which the firsttorque reducing means 102 is designed to perform is the fundamental wayto control the engine output torque. Therefore, the first torquereducing means 102 is superior in drivability (or ability of providingnormal and stable combustion) and fuel economy to the other torquereducing means. For example, the use of the brake system deterioratesthe fuel economy, and the engine output reduction of the fuel cut offcontrol is stepwise, and not smooth. In this point, the first torquereducing means 102 is worthy to employ.

On a slippery road surface of a low μ, on the other hand, the controlsystem can properly reduce the engine torque with the first torquereducing means 102. If the torque reduction by the first torque reducingmeans 102 is insufficient to restrain the drive wheel slippage, thecontrol system can further reduce the engine output torque with thesecond torque reducing means 103, and thereby offer a satisfactoryperformance of traction control.

The first torque reducing means 102 can employ a simple two stepactuator (such as item 15 shown in FIG. 4) alternating between twostates. In this case, the control system is further advantageous insimplicity in construction, reliability and cost.

The road condition sensing means 101 may be arranged to determine theroad surface friction coefficient by sensing a vehicle acceleration. Thevehicle driving force enabling the vehicle to run without slippage withrespect to the road surface increases as the road surface frictioncoefficient becomes higher, and there is a proportional relationshipbetween the driving force and the acceleration. Therefore, the roadcondition sensing means 101 can detect the road surface frictioncoefficient from the acceleration at the instant of occurrence of aslip. For the same reason, the road condition sensing means 101 candetect the road surface friction coefficient by estimating the drivingforce of the drive wheels.

The second driving torque reducing means 103 may take the form of ameans for reducing the engine output torque stepwise by controlling afuel cut cylinder number. In this case, the second torque reducing means103 can determine a desired reduction quantity of a fuel supply quantityin accordance with a slip deviation of a slip quantity sensed by theslip condition sensing means 100 from a desired slip quantity; thendetermine a desired fuel cut cylinder number in accordance with thereduction quantity; and control the actual fuel cut cylinder number inaccordance with the desired fuel cut cylinder number. In this way, thecontrol system can accurately control the reduction of the driving forcein accordance with the slip condition.

The torque reduction limiting means 105 may be in the form of a simplearrangement which limits the operation of the first torque reducingmeans 102 by increasing a slip threshold (such as SR1 shown in FIG. 8)in accordance with the friction condition (such as μ), and prevents theoperation of the first torque reducing means 102 when the slip quantitysensed by the slip condition sensing means 100 is greater than the slipthreshold.

The torque reduction limiting means 105 may be arranged to limit theoperation of the first torque reducing means 102 by increasing theamount of torque reduction achieved by the second torque reducing means103.

According to another aspect of the present invention, as shown in FIG.2, a driving torque control system comprises; (i) a second throttlevalve 104 which is disposed in series to a first throttle valve in anintake passage for an engine; (ii) a slip condition sensing means 100for sensing a drive wheel slip condition of the vehicle; (iii) a torquereducing means 102 for reducing a vehicle driving torque of the vehicleby closing said second throttle valve to the closed position inaccordance with the slip condition sensed by said slip condition sensingmeans; (iv) a required engine output sensing means 107 for sensing arequired engine output (such as TR shown in FIG. 10); and (v) a torquereduction inhibiting means 109 for inhibiting an operation of saidtorque reducing means 102 when said required engine output is equal toor greater than a predetermined level.

If the current engine output torque is high, the torque reductioninhibiting means 109 judges the driver's demand for acceleration to behigh, and inhibits the operation of the torque reducing means 102 eventhough the slip condition sensing means 100 detects the slippingcondition requiring the torque reduction of the torque reducing means102. Therefore, this control system enables an accelerating operationwith either or both of the drive wheels slipping, and helps escape forma stuck condition.

The required engine output sensing means 107 can readily sense therequired engine output representing the driver's demand for accelerationby sensing a manipulated quantity of the accelerator of the vehicle. Therequired engine output sensing means 107 may be arranged to determinethe required engine output in accordance with a fuel supply quantitysupplied to the engine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing, as an example, an arrangement ofvarious means appearing in first and second embodiments of the presentinvention.

FIG. 2 is a block diagram showing, as an example, an arrangement ofvarious means appearing in a third embodiment of the present invention.

FIG. 3 is a schematic view showing a vehicle equipped with an enginedriving torque control system according to the first, second or thirdembodiment of the present invention.

FIG. 4 is a schematic view showing an actuating system for a secondthrottle valve 4 shown in FIG. 3.

FIG. 5 is a time chart showing operating times of the second throttlevalve actuating system of FIG. 4.

FIG. 6 is a flow chart showing a control procedure performed by acontrol system according to a first practical example of the firstembodiment.

FIG. 7 is a flow chart showing a section of a control procedureaccording to a second practical example of the first embodiment.

FIG. 8 is a flow chart showing a section of a control procedureaccording to a first practical example of the second embodiment.

FIG. 9 is a flow chart showing a section of a control procedureaccording to a second practical example of the second embodiment.

FIG. 10 is a flow chart showing a control procedure performed by acontrol system according to one practical example of the thirdembodiment of the present invention.

FIG. 11 is a graph showing an operation of the control system accordingto the third embodiment.

FIG. 12 is a graph showing an operation of a traction control of asimple type.

FIG. 13 is a block diagram showing, as an example, an arrangement ofvarious means used in the first, second and third embodiments.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 3 and 4 show component parts employed in illustrated embodimentsof the present invention.

A first throttle valve 3 is provided in an intake passage 2 of anengine 1. The engine 1 of this example is transversely mounted on an FFvehicle. The first throttle valve 3 is connected with an acceleratorpedal so that the opening degree of the first throttle valve 3 isdetermined by the depression degree of the accelerator pedal.

A second throttle valve 4 is further provided in the intake passage 2 ofthe engine 1. The second throttle valve 3 of this example is disposed onthe upstream side of the first throttle valve 3. The second throttlevalve 3 is a normally open valve.

An fuel injector valve 31 is provided for each cylinder of the engine.

Wheel speed sensors 5A and 5B are provided for right and left frontwheels (drive wheels) of the vehicle. Wheel sensors 6A and 6B are forright and left rear wheels (non-drive wheels).

A traction control unit 7 receives wheel speed signals from the wheelspeed sensors 5A, 5B, 6A and 6B, detects an occurrence of slip inaccordance with a wheel speed difference between a drive wheel speed anda non-drive wheel speed, and produces driving force reduction requestsignals when the slip occurs. A first one of the driving force reductionrequest signals is sent to an actuating system for actuating the secondthrottle valve 4 (to a three way solenoid valve 21, to be exact).

An engine control module 8 receives a second one of the driving forcereduction request signals from the traction control unit 7, and performsa fuel cut control so that fuel supply is cut off to a predeterminednumber of engine cylinders determined in accordance with a requiredreduction of the vehicle driving torque.

A longitudinal acceleration (G) sensor 32 shown in FIG. 3 senses thelongitudinal acceleration Xg of the vehicle.

A lateral acceleration (G) sensor 33 senses a lateral acceleration Yg ofthe vehicle. The traction control unit 7 receives the signals from thelongitudinal and lateral G sensors 32 and 33, and estimates a roadsurface friction coefficient μ.

A throttle sensor 34 senses the opening degree of the first throttlevalve 3. The signal representing the throttle opening sensed by thethrottle sensor 34 is supplied, as an input signal for engine control,to the engine control module 3, and further sent from the engine controlmodule 8 to the traction control unit 7, as a signal for discriminatinga condition to inhibit the closing operation of the second throttlevalve 4 as mentioned later.

The actuating system for the second throttle valve 4 is shown in FIG. 4.

A butterfly type valve is employed as the second throttle valve 4 inthis example. The butterfly type second throttle valve 4 is disposed ina chamber 10 of resin forming the intake passage 2.

A valve shaft 11 of the second throttle valve 4 is eccentric, andsituated to one side with respect to the center line of the chamber 10.

A lever 12 for operating the second throttle valve 4 is fixed to thevalve shaft 11. The lever 12 is swingable between stoppers 13 and 14 forlimiting the swing movement of the lever 12. With the aid of thestoppers 13 and 14, the second throttle valve 4 can assume a fully openposition and a closed position of a predetermined small opening. In theclosed position, the opening degree of the second throttle valve 4 ofthis example is about 1/8.

A negative pressure diaphragm type actuator 15 shown in FIG. 4 includesa diaphragm 16 which is connected with the lever 12 of the secondthrottle valve 4 by an output rod 17. The diaphragm type actuator 15further includes a negative pressure working chamber 18 defined by thediaphragm 16, and a spring 19 disposed in the negative pressure chamber18, for urging the diaphragm 16. When a negative pressure is introducedinto the negative pressure chamber 18, the diaphragm 16 moves to theright as viewed in FIG. 4, and brings the second throttle valve 4 to theclosed position. Introduction of the atmospheric pressure into thenegative pressure chamber 18 causes a leftward displacement of thediaphragm 16, and accordingly, the second throttle valve 4 returns tothe fully open position.

A three way solenoid valve 21 is connected with the negative pressureworking chamber 18 of the diaphragm type actuator 17 by a communicationpassage 20, and arranged to selectively introduce the negative pressureor the atmospheric pressure into the negative pressure chamber 18. In anoff state, the three way solenoid valve 21 closes a negative pressureinlet port P1 and opens an atmospheric pressure inlet port P2. In an onstate, the atmospheric pressure inlet port P2 is closed, and thenegative pressure inlet port P1 is opened.

A negative pressure tank (or reservoir) 23 and a check valve 24 areprovided in a passage for introducing the negative pressure from theintake manifold to the three way solenoid valve 21. A negative pressurefeed passage 22 fluidly connects the negative pressure port P1 of thethree way solenoid valve 21 with the negative pressure tank 23. Throughthe check valve 24, the negative pressure tank 23 is further connectedto the intake manifold (to the position downstream of the first throttlevalve 3 in the intake passage 2).

An orifice 26 is provided in an atmospheric pressure passage 25 forintroducing the atmospheric pressure to the atmospheric pressure port P2of the three way solenoid valve 21. The orifice 26 serves as anoperating time differentiating means.

The use of the thus-constructed negative pressure diaphragm typeactuator system for switching the second throttle valve 4 between thefully open state and the predetermined fully closed state in a manner ofa two step control can significantly reduce the manufacturing cost ascompared with the conventional system using the DC motor. In place ofthe negative pressure diaphragm type actuator, it is optional to employa solenoid actuator capable of providing a predetermined stroke. Thesolenoid actuator is also advantageous in cost as compared with the DCmotor.

The driving torque control system constructed as shown in FIGS. 3 and 4is operated as follows:

In normal operations, the three way solenoid valve 21 is held in the offstate. Accordingly, the negative pressure inlet port P1 is closed, andthe atmospheric pressure inlet port P2 is open. In this state, theatmospheric pressure is introduced into the negative pressure chamber18, and the spring 19 holds the second throttle valve 4 in the fullyopen position by pushing the diaphragm 16 leftward in FIG. 4. The fullyopen position corresponds to an opening degree which can ensure anamount of air required by the engine.

The traction control unit 7 of this example computes the wheel speeddifference between the front wheel speed determined from the signals ofthe front wheel speed sensors 5A and 5B and the rear wheel speeddetermined from the signals of the rear wheel speed sensors 6A and 6B.When this wheel speed difference exceeds a predetermined value, thetraction control unit 7 judges a slip condition present, produces theengine driving torque reduction request signal, and delivers thisreduction request signal to the three way solenoid valve 21

By this engine driving torque reduction request signal, the three waysolenoid valve 21 switches from the off state to the on state, and makesthe atmospheric pressure port P2 closed and the negative pressure portP1 open. As a result, the negative pressure is introduced into thenegative pressure working chamber 18 of the diaphragm unit 15, andcauses the diaphragm 16 to move rightward in FIG. 4 against theresilient force of the spring 19. With this rightward displacement ofthe diaphragm 16, the second throttle valve 4 is brought to the closedposition of an approximately 1/8 opening degree.

The opening degree of the second throttle valve 4 in the closed positionis set to a predetermined small degree which is required to repress theslip, but which is not so small as to cause an engine stall. The settingto about 1/8 is adequate to repress the slip without causing an enginestall.

A closing time of the second throttle valve 4 is set to a very shorttime equal to or smaller than 0.2 sec in order to repress the slipquickly on a low μ road. The closing time (t1 in FIG. 5) is a period oftime during which the second throttle valve 4 is moving from the fullyopen position to the closed position. The effective pressure receivingarea of the diaphragm 16, the diameter of the negative pressure feedpassage 22 and the like are so chosen as to decrease the closing time ofthe second throttle valve 4.

The closing operation of the second throttle valve 4 decreases thedriving force of the engine, and lowers the degree of slip. When thedegree of slip is lowered sufficiently, the traction control unit 7cancels the driving torque reduction request signal.

The three way solenoid valve 21 is thus deprived of the reductionrequest signal, and returns from the on state to the off state.Therefore, the negative pressure inlet port P1 closes and theatmospheric pressure inlet port P2 opens and introduces the atmosphericpressure into the negative pressure working chamber 18 of the diaphragmunit 15. The diaphragm 16 moves leftward in FIG. 4 by the force of thespring 19 and returns the second throttle valve 4 to the fully openposition.

An opening time required for the second throttle valve 4 to move fromthe closed position to the fully open position (t2 shown in FIG. 5) isset equal to more than ten times as much as the closing time (t1) fromthe fully open position to the closed position, in order to prevent anundesired feeling of dashing when the road changes from a low μcondition to a high μ condition. In this example, the closing time (t2)is set to 2˜7 sec by making a flow passage resistance of the atmosphericpressure passage 25 greater with the orifice 26 than that of thenegative pressure feed passage 22.

The diaphragm type actuator 15 is designed to hold the second throttlevalve 4 in the fully open position even if the negative pressuredisappears for one reason or another. In this case, the pressure in thenegative pressure chamber 18 becomes equal to the atmospheric pressure,and the second throttle valve 4 is put in the fully open position.Therefore, this control system allows the vehicle to be operated even insuch a case.

With the eccentric or asymmetric arrangement, the second throttle valve4 remains open even if the second throttle valve 4 becomes loose, freelyrotatable or disconnected from the output rod 17, for example. The valveshaft 11 is deviated from the center of the intake passage 2, and thepressure receiving area of the valve element on one side of the valveshaft 11 is greater than that on the other side of the valve shaft 11.Therefore, the pressure difference between the upstream side of thesecond throttle valve 4 and the downstream side (receiving a negativepressure) produces a moment in the direction to open the second throttlevalve 4. This control system allows the vehicle to be operated even inthis case.

FIG. 6 shows a control procedure performed by a driving torque reductioncontrol (or traction control) system according to a first practicalexample of a first embodiment of the present invention. This controlsystem combines the above-mentioned first driving force reducing means102 for reducing the vehicle driving force by controlling the secondthrottle valve 4 and the second driving force reducing means 103 which,in this example, reduces the vehicle driving force by controlling thenumber of fuel cut cylinders.

At a step S1 of FIG. 6, the control system determines whether or not aslip exists.

If no slip is detected at the step S1, the control system maintains thefully open state of the second throttle valve 4 at a step S8, and thenterminates this routine. If a slip is detected, the control systemproceeds to a step S2.

In this example as mentioned before, the control system detects anoccurrence of slip by monitoring the wheel speed difference between thefront and rear wheel speeds. The control system determines a slipquantity S which is a wheel speed difference resulting from subtractionof the nondrive wheel speed from the drive wheel speed. For example, thedrive wheel speed is an arithmetic mean of the left and right drivewheel speeds, and the nondrive wheel speed is an arithmetic mean of theleft and right nondrive wheel speeds. In this example, the controlsystem determines, at the step S1, whether the sensed actual slipquantity S in the form of the sensed wheel speed difference is equal toor greater than a predetermined slip quantity SR0. If it is, then thecontrol system judges that a slip condition exists, and proceeds to thestep S2. If the sensed slip quantity S is smaller than the predeterminedslip quantity SR0, then the control system proceeds from the step S1 tothe step S8.

If an occurrence of slip is detected, the control system computes a roadsurface friction coefficient μ at the step S2. In this example, thecontrol system determines the friction coefficient μ in accordance withthe longitudinal acceleration Xg sensed by the longitudinal accelerationsensor (longitudinal G sensor) 32 and the lateral acceleration Yg sensedby the lateral acceleration sensor (lateral G sensor) 33. For instance,a formula for calculating the friction coefficient μ is; μ=k·(Xg²+Yg²)1/2 where k is a constant. In this case, the friction coefficient μis proportional to the magnitude of the acceleration in the travelingdirection of the vehicle. The longitudinal acceleration sensor 32, thelateral acceleration sensor 33 and the means for performing the step S2serve as the road condition sensing means 101.

It is possible to calculate the longitudinal and lateral accelerationsXg and Yg without using the acceleration sensors 32 and 33. Forinstance, the longitudinal acceleration Xg can be calculated from theacceleration of the nondrive wheel speed (or the rate of change of thenondrive wheel speed with respect to time, which can be determined bythe amount of change of the nondrive wheel speed during a unit timeinterval). The lateral acceleration Yg can be calculated from thedifference between the wheel speeds of the left and right nondrivewheels. That is, Yg=|VFL-VFR|·(VFL-VFR)/2·α where VFL and VFR are theleft and right nondrive wheel speeds and α is a constant.

At a step S3, the control system compares the friction coefficient 82obtained at the step S2 with a predetermined value μ0, and determineswhether the friction coefficient μ is smaller than or equal to thepredetermined value μ0. The predetermined friction coefficient value μ0is set equal to a value (about 0.35) corresponding to the level of apressed-snow-covered road surface, for instance. Besides, the openingdegree (1/8) of the second throttle valve 4 in the predetermined closedposition is set approximately equal to an upper limit of the range inwhich, on a road surface having the predetermined coefficient value μ0,the vehicle can be operated at a restrained degree of slip withoutlosing a grip.

A step S4 is reached if the sensed road surface friction coefficient μis judged to be equal to or smaller than the predetermined value μ0. Atthe step S4, the control system brings the second throttle valve 4 tothe predetermined closed position by turning on the three way solenoidvalve 21. The actuator for the second throttle valve 4 and the means forperforming the step S4 serve as the first driving force reducing means102.

Thus, this control system can reduce the engine output torque and hencethe vehicle driving force to a level enabling the vehicle to travel witha gripping ability on the road surface of the predetermined frictioncoefficient μ0.

At steps S6 and S7 following the step S4, the control system performsthe second driving force reduction control to control the number of thefuel cut cylinders in accordance with the slip condition. The means forperforming the steps S6 and S7 serves as the second driving forcereeducating means 103.

In this example, the control system determines, at the step S6, adesired fuel cut quantity C from a deviation ε of the sensed slipquantity S from a preset desired slip quantity Stg for enabling anoptimum driving, according to a PID control law (or control action).Then, at the step S7, the control system performs the fuel cut controlso that the number of the fuel cut cylinder or cylinders corresponds tothe desired fuel cut quantity C determined at the step S6. The desiredfuel cut quantity C in this example is given by; ##EQU1## where KP is aproportional gain of the proportional control action, KI is an integralgain of the integral control action, and KD is a derivative gain of thederivative control action.

When the sensed actual road surface friction coefficient μ is equal toor smaller than the predetermined value μ0, the amount of slip exceedsthe desired slip level, and the control system performs a feedbackcontrol for reducing the vehicle driving force by reducing the supply offuel, and thereby provides the vehicle with an adequate amount ofdriving force while holding the slip quantity around the desired sliplevel.

A step S5 is reached if the sensed road surface friction coefficient μis judged to be greater than the predetermined value μ0. The controlsystem maintains the fully open state of the second throttle valve 14 atthe step S5, and then proceeds to the step S6. The means for performingthe step S3 and the step S5 corresponds to the driving force reductioncontrol limiting means 105.

Then, at the step S6 and S7 following the step S5, the control systemdetermines the desired fuel cut quantity C according to the PID controllaw, and cuts off the supply of fuel to one or more cylinders the numberof which corresponds to the desired fuel cut quantity C as describedbefore. In this case, the control system holds the second throttle valve4 fully open, and controls the vehicle driving force only by the fuelcut control. That is, this control system refrains the driving forcereduction control by the second throttle valve 4 because, on a roadsurface having a friction coefficient μ higher than the predeterminedfriction coefficient μ0, the closing of the second throttle valve 4results in an excessive reduction of the vehicle driving force, so thatthe driving operation is limited to a low vehicle speed operation with aslip rate lower than the desired level. When the vehicle is relieved ofthe slip condition, and accelerated again, the accelerating performanceof the vehicle is made insufficient by a considerable delay of anincrease of the engine output due to a delay from the switchingoperation of the second throttle valve 4 from the closed position to thefully opening position, to an actual increase of intake air flow rate.

Therefore, the control System of this example prevents the operation ofthe first torque reducing means 102, and controls the slip only with thesecond torque reducing means 103 on such a high friction coefficientroad surface. The control system can allows a relatively high vehiclespeed operation while keeping the slip quantity around the desiredlevel, and at the same time provide a responsive reacceleratingperformance after escape from the slip condition by causing the drivingtorque to be increased quickly by cancellation of the operation of thesecond torque reducing means 103.

The second driving force reducing means 103 is a means which is superiorin response characteristic or speed of response to the first drivingforce reducing means 102. The second driving force reducing means 103 isin the form of the means for fuel cut control in the example of FIG. 6.As the second driving force reducing means 103, it is possible, however,to employ some other means capable of reducing the vehicle drivingforce. For instance, the second driving force reducing means 103 may bea controlling means or system for controlling the engine output bycontrolling an engine operating parameter other than the throttleopening, such as a retard angle of the ignition timing, or a shiftcontrolling means or system for an automatic transmission, or acontrolling means or system for increasing a braking force. In the caseof the ignition timing control, the combustion becomes worse if theignition timing is retarded too much. Therefore, it is desirable to usethe retard angle control in combination with the fuel cut control, andto reduce the engine power output finely and precisely. In the case ofthe automatic transmission shift down control or the brake forcecontrol, the control system may be arranged to calculate a desireddriving force reduction quantity corresponding to the before-mentioneddesired fuel cut quantity C, and to carry out a control operation tocontrol the transmission ratio (or gear ratio) or the braking force to alevel determined by the calculated desired driving force reductionquantity.

In the example shown in FIG. 6, the control system determines the roadsurface friction coefficient μ from the acceleration of the vehicle.However, it is optional to determine the friction coefficient μ from thedriving force of the drive wheels of the vehicle when the slip occurs,as shown in FIG. 7.

FIG. 7 shows a sequence of steps which can be substituted for theprogram section of the steps S2 and S3 in the flow chart of FIG. 6.

The control system proceeds to a step S11 of FIG. 7 from the step S1 ifthe answer of the step S1 of FIG. 6 is affirmative. At the step S11, thecontrol system estimates an engine output torque TE at the time of slipoccurrence, from the basic fuel injection quantity TP or the like.

At a step S12, the control system computes a drive wheel output torqueTD by multiplying the engine output torque TE determined at the step S11by a transmission gear ratio NT of the transmission, and a final gearratio NF of the final reduction gear. That is, TD=TE×NT×NF.

At a step S13, the control system compares the drive wheel output torqueTD with a predetermined value TS. If TD is equal to or smaller than TS(TD≦TS), the control system proceeds from the step S13 to the step S4shown in FIG. 6. By following the course of the stepsS1→S11→S12→S13→S4→S6→S7, the control system closes the second throttlevalve 4 and performs the fuel cut control. If TD is greater than TS(TD>TS), the control system takes the course of the stepsS1→S11→S12→S13→S5 →S6→S7, and performs only the fuel cut control bykeeping the second throttle valve 4 in the fully open state.

The predetermined drive wheel output torque value TS is set equal to avalue corresponding to the magnitude of the drive wheel output torqueenabling a vehicle operation at a restrained degree of slip withoutlosing a grip. Therefore, a driving force control provided by the stepsS11˜S13 is substantially identical to the control depending on thecomparison in the steps S2 and S3 between the friction coefficient μ andthe predetermined value μ0.

FIG. 8 shows a first practical example of a second embodiment of thepresent invention. A sequence of steps shown in FIG. 8 can besubstituted for the step S3 in the control procedure of FIG. 6. In theexample of FIG. 8, the decision step S3 to select one of the alternativesteps S4 and S5 is replaced by a step S22 in which the slip quantity Sis compared with a predetermined slip threshold value SR1, and thisthreshold value SR1 is varied at a step S21 in accordance with thefriction coefficient μ determined in the step S2 to limit the operationof the first driving force reducing means 102.

After the determination of the friction coefficient μ at the step S2 inFIG. 6, the control system proceeds to the step S21, and determines, atthe step S21, the threshold SR1 in dependence on the frictioncoefficient μ. As shown in a graph in FIG. 8, the control systemincreases the threshold SR1 as the friction coefficient μ increases. Inthis example, the threshold SR1 is a monotone increasing function(monotone nondecreasing function) of μ. More specifically, in thisexample, SR1 is fixed at a predetermined value when μ is smaller than apredetermined value and SR1 is increased linearly with μ when the μ isgreater than the predetermined value.

At the step S22, the control system compares the sensed slip quantity Swith the threshold SR1 determined at the step S21. If S is greater thanSR1 (S>SR1), the control system proceeds from the step S22 to the stepS4 of FIG. 6, and performs the closing operation of the second throttlevalve 4 at the step S4, and the fuel cutting operation of the steps S6and S7. If the sensed slip quantity S is equal to or smaller than SR1(S≦SR1), then the control system proceeds from the step S22 to the stepS5 of FIG. 6, and performs the operation to hold the second throttlevalve 4 fully open at the step S5 and the fuel cutting operation at thesteps S6 and S7.

In this way, the control system limits the closing operation of thesecond throttle valve 4 when the friction coefficient μ is high, bydecreasing the likelihood of selection of the step S4 in accordance withthe friction coefficient μ. When the closing of the second throttlevalve 4 is prevented at the step S5, the control system performs thedriving force reduction control by the fuel cut control or some otherform of the second driving force reducing means 103.

In the example of FIG. 8, the threshold SR1 is equal to or greater thanSR0 used in the step S1.

FIG. 9 shows a second practical example according to the secondembodiment. A sequence of steps shown in FIG. 9 can be substituted forthe step S3 in the control procedure of FIG. 6. In the example of FIG.9, the control system is arranged to limit the operation of the firstdriving force reduction control by varying the degree of the operationof the second driving force reduction control. More specifically, thecontrol system increases the control gain, such as KP, of the seconddriving force reduction control such as the fuel cut control, as thefriction coefficient μ increases.

At a step S31 shown in FIG. 9, the control system determines the controlgain used in the fuel cut control in accordance with the frictioncoefficient μ determined at the step S2 of FIG. 6. The control gain inthis example is the proportional gain KP used for calculating thedesired fuel cut quantity C according to the PID control law in the stepS6. The proportional control gain KP is increased as the frictioncoefficient μ increases. The control gain in this example is a monotoneincreasing (or nondecreasing) function of μ, and increases with μ in thesame manner as the threshold SR1 of the step S21 of FIG. 8.

At a step S32 of FIG. 9, the control system compares the sensed slipquantity S with a predetermined threshold value SR1. In the example ofFIG. 9, SR1 is a predetermined constant greater than SR0. If the sensedslip quantity S is greater than SR1 (S>SR1), the control system performsthe closing operation of the second throttle valve 4 at the step S4 ofFIG. 6, and the fuel cut control operation at the steps S6 and S7. If Sis equal to or smaller than SR1 (S≦SR1), then the control systemperforms the operation to maintain the fully open position of the secondthrottle valve 4 at the step S5, and the fuel cut control operation atthe steps S6 and S7.

In this way, the control system increases the control gain KP withincrease in the friction coefficient μ, and thereby increases thedesired fuel cut quantity C even if the sensed slip quantity S remainsunchanged. With the increased fuel cut quantity C, the control systemincreases the number of the cylinders to which the fuel supply is cutoff, and efficiently decreases the vehicle driving force to such a levelas to restrain the slipping condition. As a result, the control systemcan relatively decrease the likelihood of the second throttle valveclosing operation of the step S4 as in the preceding example.

FIG. 10 shows a control procedure according to a third embodiment of thepresent invention. A control system according to the third embodiment isconstructed substantially in the same manner as the control system shownin FIG. 3. In the third embodiment, however, the second driving forcereducing means 103 can be omitted.

At a step S41 of FIG. 10, the control system checks whether a slipcondition exists (as in the step S1 of FIG. 1). If no slip is detected,the control system proceeds to a step S47 and maintain the fully openstate of the second throttle valve 4.

If the slip condition exists, the control system proceeds to a step S42and determines, at the step S42, a required engine output torque TR,which, in this example, is an opening degree θ of the first throttlevalve 3. The means for performing the step S42 corresponds to therequired engine output torque sensing means 107.

At a step S43, the control system determines whether a driver's inputcommand to accelerate the vehicle is greater or not, by determiningwhether the required engine output torque TR is equal to or greater thana predetermined value TR0.

If TR≧TR0, that is, throttle opening μ is equal to or greater than apredetermined throttle opening value θ0 (θ≧θ0), then the control systemproceeds to the step S47, and maintains the fully open state of thesecond throttle valve 4. In this case, the control system performsneither the second throttle closing operation nor the fuel cut controloperation.

When, for example, the vehicle is stuck, and the degree of the driver'sintention to accelerate is high to escape from the stuck condition, thiscontrol system increases the vehicle driving force to accelerate theengine, allowing a drive wheel slippage, in compliance with the driver'saccelerating intention, and provides the vehicle with an acceleratingability to escape from the stuck condition.

If TR<TR0, that is, θ<θ0 (the driver's accelerator input command issmall), the control system proceeds from the step S43 to a step S44. Thecontrol system closes the second throttle valve 4 to the predeterminedclosed position at the step S44, and then performs the fuel cut controloperations at steps S45 and S46 similar to the steps S6 and S7 shown inFIG. 6. In this case, it is optional to select the closing or openingoperation of the second throttle valve 4 in accordance with the level ofthe slip as in the preceding embodiments.

In the example of FIG. 10, the required engine output torque TR is inthe form of the throttle opening degree θ of the first throttle valve 3.It is, however, possible to employ the basic fuel injection quantity TP,an intake pressure or some other condition. For example, the basic fuelinjection quantity Tp is a quantity obtained by multiplying a fractionwhose numerator is the intake air flow rate Qa and whose denominator isthe engine speed Ne, by a predetermined constant K as in a conventionalexample of the fuel injection control. In this case, Tp=K×(Qa/Ne).

FIG. 11 shows characteristics of the control system of the example shownin FIG. 10.

According to the illustrated embodiments of the present invention, asexplained above, the vehicle driving torque control system is made up ofthree parts which are:

an actuating means for reducing a vehicle driving torque or force inresponse to a control signal;

a vehicle operating condition sensing means for sensing one or morevehicle operating conditions; and

a controlling means for controlling the vehicle driving torque byproducing the control signal.

FIG. 13 shows, as one of various possible examples, various componentswhich can constitute these three means.

The actuating means in the example of FIG. 13 comprises a firstactuating means 511 (items 4, 15 and 21, for example) and a secondactuating means 512 (item 31 and/or 8, for example).

The vehicle operating condition sensing means in the example of FIG. 13comprises a first sensing means 521 (items 5A, 5B, 6A and 6B, forexample), and a second sensing means 523 (32 and 33, or 34 for example).

The controlling means in the example shown in FIG. 13 comprises:

a first control means 531 for controlling the vehicle driving torque byoperating the first actuating means 511;

a second control means 532 for controlling the vehicle driving torque byoperating the second actuating means 512;

a slip detecting means (541, 542); and

a torque reduction restraining means (552, 554, 555, 556).

The slip detecting means in the example of FIG. 13 comprises:

a first parameter determining means 541 for determining a first vehicleoperating parameter such as a drive wheel slip quantity S from thesignals from the first sensing means 521; and

a slip detecting comparator means 542 for producing a torque reductionrequest signal when the slip quantity S is greater than a predeterminedslip level (such as SR0).

The torque reduction restraining means in the example of FIG. 13comprises either or both of:

a first set of a second parameter determining means 551 for determininga second operating parameter such as a parameter representing a roadsurface friction coefficient μ or a drive wheel output torque TD, and acondition discriminating means 552 in the form of a partial disablingmeans for disabling the first control means 531 when the secondparameter (μ, TD)is greater than a predetermined value (μ0, TS); and

a second set of a further (second) parameter determining means 553 fordetermining a further (second) operating parameter such as a variable(TR) representing an accelerator opening, or some other variablerepresenting a driver's intention of accelerating the engine, and acondition discriminating means 554 in the form of a total disablingmeans for disabling both of the first and second control means 531 and532 when the further (second) operating parameter TR is greater than apredetermined value (TR0).

The second sensing means 522 in one example may comprise thelongitudinal and lateral acceleration sensors 32 and 33 as in the firstexample of the first embodiment shown in FIG. 6.

The second sensing means 522 in another example may comprise one or moreengine operating condition sensors for determining a variable (such asTE) representing an engine output torque by sensing one or more engineoperating conditions such as an engine load condition (such as Qa) andan engine speed condition (such as Ne); and a transmission sensor fordetermining a transmission ratio (such as NT). The second sensing means522 may further comprise a means for determining a basic fuel injectionquantity TP (in a fuel injection control system). Alternatively, thesecond sensing means 522 may be arranged to receive information items,such as the basic fuel injection quantity TP and the transmission ratio,from some other vehicle control system such as a fuel injection controlsystem and/or a transmission control system, or from a sensor or sensorsfor the other vehicle control system.

The second sensing means 522 in still another example may comprise afirst sensor means (32 and 33, for example) for sensing a road conditionindicative vehicle operating condition to determined a road conditionindicative vehicle operating parameter (μ or TD, for example), and asecond sensor means (34, for example) for sensing a driver's acceleratorinput indicative vehicle operating condition (θ, for example) todetermine an accelerator input indicative parameter (TR, for example);and the reduction restraining means may comprise a first restrainingmeans (552) for limiting the torque reduction control operation of thefirst control means 531 when the road condition indicative operatingparameter (μ, TD) is increased, and a second retraining means (554) forlimiting the torque reduction control operation of the first controlmeans 531 when the accelerator input indicative operating parameter (TR)is increased. In this case, the second vehicle operating conditioncomprises the road condition indicative condition and the acceleratorinput indicative condition, and the second vehicle operating parametercomprises the road condition indicative parameter and the acceleratorinput indicative parameter.

The slip condition sensing means 100 shown in FIG. 1 or 2 can correspondto the first sensing means 521 and the first parameter determining means541. The first torque reducing means 102 can correspond to the firstcontrol means 531 and the first actuating means 511 excluding the secondthrottle valve (4, 104). The second torque reducing means 103 cancorrespond to the second actuating means 512 and the second controlmeans 532. The road condition sensing means 101 can correspond to thesecond sensing means 522 and the second parameter determining means 551.The torque reduction limiting means 105 shown in FIG. 1 can correspondto the partial disabling means 552. The torque reduction inhibitingmeans 109 can correspond to the total disabling means 554.

What is claimed is:
 1. A driving torque control system for a vehicle,comprising:a second throttle valve which is disposed in series to afirst throttle valve in an intake passage for an engine and which isswitched between a fully open position and a predetermined closedposition; a slip condition sensing means for sensing a drive wheel slipcondition of the vehicle; a first torque reducing means for reducing avehicle driving torque of the vehicle by closing said second throttlevalve to the predetermined closed position in accordance with the slipcondition sensed by said slip condition sensing means; a second torquereducing means for reducing the vehicle driving torque in accordancewith the slip condition sensed by said slip condition sensing means; aroad condition sensing means for sensing a frictional condition of aroad surface under the vehicle; and a limiting means for limiting anoperation of said first torque reducing means and allowing an operationof said torque reducing means in a road surface condition of a frictioncoefficient higher than a predetermined friction level, wherein saidfirst torque reducing means comprises a two-step actuator for switchingsaid second throttle valve from the fully open position to thepredetermined closed position to reduce the vehicle driving torque, andwherein said second torque reducing means comprises means for reducingthe vehicle driving torque by altering a manipulated variable which isone of a fuel cut cylinder number that is a number of engine cylindersto which fuel supply is to be cut off, a retard angle in an ignitiontiming of an ignition system of the engine, a braking force produced bya brake system of the vehicle and a transmission ratio of a transmissionof the vehicle.
 2. A control system as claimed in claim 1 wherein saidactuator comprises a negative pressure working chamber for receiving anegative pressure, and a diaphragm for moving said second throttle valvebetween the fully open position and the predetermined closed position independence on whether the negative pressure is introduced to saidnegative pressure working chamber or not.
 3. A control system as claimedin claim 1 wherein said road condition sensing means comprises anacceleration sensing means for sensing a vehicle acceleration which isat least one of a longitudinal acceleration and a lateral accelerationof the vehicle, and a friction coefficient determining means fordetermining a road surface friction coefficient in accordance with amagnitude of the vehicle acceleration sensed when a slip occurs.
 4. Acontrol system as claimed in claim 1 wherein said road condition sensingmeans comprises a driving force estimating means for estimating adriving force of drive wheels of the vehicle, and determining a roadsurface friction coefficient in accordance with a magnitude of thedriving force estimated when a slip occurs.
 5. A control system asclaimed in claim 1 wherein said second torque reducing means is a meansfor reducing the vehicle driving torque independently of said firsttorque reducing means, and comprises a fuel cutting means for varying anactual fuel cut cylinder number in accordance with the slip condition,and said fuel cut cylinder number is a number of engine cylinders towhich a supply of fuel is cut off.
 6. A control system as claimed inclaim 5 wherein said second torque reducing means comprises a fuel cutcylinder number determining means for computing a desired reductionquantity of a fuel supply quantity in accordance with a slip deviationof a slip quantity sensed by said slip condition sensing means from adesired slip quantity, for determining a desired fuel cut cylindernumber in accordance with said reduction quantity and for commandingsaid fuel cutting means to vary the actual fuel cut cylinder number sothat the actual fuel cut cylinder number becomes equal to the desiredfuel cut cylinder number.
 7. A control system as claimed in claim 1wherein said limiting means comprises a threshold increasing means forlimiting the operation of said first torque reducing means by increasinga slip threshold in accordance with the friction condition andpreventing the operation of said first torque reducing means when theslip quantity sensed by said slip condition sensing means is greaterthan the slip threshold.
 8. A control system as claimed in claim 7wherein said limiting means comprises a torque reduction increasingmeans for limiting the operation of said first torque reducing means byincreasing a torque reduction quantity of said second torque reducingmeans.
 9. A driving torque control system for a vehicle, comprising:anactuating section for reducing a vehicle driving torque of the vehiclein response to a control signal, said actuating section comprising asecond throttle valve disposed, in an intake passage of an engine, inseries to a first throttle valve operatively connected with anaccelerator of the vehicle; a vehicle operating condition sensingsection which comprises a first operating condition sensor for sensing afirst vehicle operating condition of the vehicle to determine a firstvehicle operating parameter representing a drive wheel slip condition ofthe vehicle; and a controlling section for controlling the vehicledriving torque in accordance with the first vehicle operating parameterby sending the control signal to said actuating section, saidcontrolling section being configured to reduce the vehicle drivingtorque by closing said second throttle valve in accordance with saidfirst vehicle operating parameter, wherein said vehicle operatingcondition sensing section further comprises a second operating conditionsensor for sensing a second vehicle operating condition to determine asecond vehicle operating parameter distinct from said first vehicleoperating parameter, said second vehicle operating parameterrepresenting a road surface frictional condition, wherein saidcontrolling section is further configured to restrain a controloperation of closing said second throttle valve when said secondoperating parameter increases; wherein said actuating section comprisesfirst and second actuators for reducing the vehicle driving torque, independence on the first vehicle operating parameter, by altering firstand second manipulated variables, respectively, said first manipulatedvariable representing an opening degree of said second throttle valve,said second manipulated variable representing one of a fuel cut cylindernumber that is a number of engine cylinders to which fuel supply is tobe cut off, a retard angle in an ignition timing of an ignition systemof the engine, a braking force produced by a brake system of the vehicleand a transmission ratio of a transmission of the vehicle, wherein saidfirst actuator for altering the first manipulated variable is a two-stepactuator for normally holding said second throttle valve in a fully openposition and switching said second throttle valve from said fully openposition to a fully closed position of a predetermined opening degree inresponse to a first control signal sent from said controlling section,and wherein said controlling section is configured to reduce the vehicledriving torque only with the second actuator as a function of the firstvehicle operating parameter when said second operating parameterincreases.
 10. A control system as claimed in claim 9:wherein saidcontrolling section further comprises a slip detecting section forproducing a torque reduction request signal when said first vehicleoperating parameter representing the drive wheel slip condition isgreater than a predetermined slip level, and said controlling sectionincludes a section for producing said first control signal to close saidthrottle valve in response to said torque reduction request signal. 11.A control system according to claim 9 wherein said controlling sectioncomprises a control unit for inhibiting a closing operation of saidsecond throttle valve by holding said two-step actuator invariably insaid first stable state regardless of said torque reduction requestsignal as long as said second operating parameter is greater than saidpredetermined value.
 12. A control system according to claim 9 whereinsaid two-step actuator has only first and second stable states andalternates between the first stable state for holding said secondthrottle valve in said fully open position and the second stable statefor holding said second throttle valve in said fully closed position.13. A control system according to claim 9 wherein said two-step actuatorcomprises a negative pressure working chamber for receiving a negativepressure, a diaphragm for moving said second throttle valve between thefully open position and the fully closed position, a negative pressureinlet port for introducing the negative pressure into the negativepressure working chamber to move said second throttle valve to the fullyclosed position, and an atmospheric pressure inlet port for introducingan atmospheric pressure into the negative pressure working chamber tohold said second throttle valve in the fully open position; and whereinsaid control unit is configured to inhibit the closing operation of saidsecond throttle valve by holding said actuator in a state in which thenegative pressure inlet port is closed and the atmospheric pressureinlet port is open.
 14. A control system according to claim 9 whereinsaid second manipulated variable is said fuel cut cylinder number andsaid second actuator comprises fuel injector valves for supplying fuelto the engine.
 15. A driving torque control system for a vehicle,comprising:an actuating means for reducing a vehicle driving torque ofthe vehicle in response to a control signal, said actuating meanscomprising a second throttle valve disposed, in an intake passage of anengine, in series to a first throttle valve operatively connected withan accelerator of the vehicle, a vehicle operating condition sensingmeans which comprises a first sensing means for sensing a first vehicleoperating condition of the vehicle to determine a first vehicleoperating parameter representing a drive wheel slip condition of thevehicle; and a controlling means for controlling the vehicle drivingtorque in accordance with the first vehicle operating parameter bysending the control signal to said actuating means, said controllingmeans comprising a first control means for reducing the vehicle drivingtorque by closing said second throttle valve in accordance with saidfirst vehicle operating parameter; wherein said vehicle operatingcondition sensing means further comprises a second sensing means forsensing a second vehicle operating condition to determine a secondvehicle operating parameter distinct from said first vehicle operatingparameter, wherein said controlling means further comprises a reductionrestraining means for restraining said first control means from closingsaid second throttle valve when said second operating parameterincreases, wherein said actuating means further comprises a two-stepactuator for normally holding said second throttle valve in a fully openposition and switching said second throttle valve from said fully openposition to a predetermined closed position in response to a firstcontrol signal sent from said controlling means; wherein saidcontrolling means further comprises a slip detecting means for producinga torque reduction request signal when said first vehicle operatingparameter representing the drive wheel slip condition is greater than apredetermined slip level, and said first control means includes a meansfor producing said first control signal to close said second throttlevalve in response to said torque reduction request signal, wherein saidsecond operating parameter is a parameter representing one of a roadsurface frictional condition and a driver's accelerating intention,wherein said actuating means comprises a first actuating means forreducing the vehicle driving torque by altering a first manipulatedvariable representing an opening degree of said second throttle valve,and a second actuating means for reducing the vehicle driving torque byaltering a second manipulated variable which is one of a fuel cutcylinder number that is a number of engine cylinders to which fuelsupply is to be cut off, a retard angle in an ignition timing of anignition system of the engine, a braking force produced by a brakesystem of the vehicle and a transmission ratio of a transmission of thevehicle, and said controlling means further comprises a second controlmeans for reducing the vehicle driving torque by operating said secondactuating means in response to said torque reduction request signal,wherein said first actuating means comprises said two-step actuatorwhich comprises a negative pressure working chamber for receiving anegative pressure, and a diaphragm for moving said second throttle valvebetween the fully open position and the predetermined closed position independence on whether the negative pressure is introduced to saidnegative pressure working chamber or not, so that said first manipulatedvariable representing the opening degree of said second throttle valvealternates between a first value corresponding to said fully openposition and a second value corresponding to said closed position,wherein said second vehicle operating condition sensed by said secondsensing means to determine said second vehicle operating parameter isone of a vehicle acceleration, an engine operating condition of theengine, and an accelerator condition which is a condition of anaccelerating system of the vehicle, and wherein said first sensing meanscomprises wheel speed sensors for sensing a drive wheel speed and anon-drive wheel speed of the vehicle to determining said first operatingparameter which is a drive wheel slip quantity resulting fromsubtraction of said non-drive wheel speed from said drive wheel speed.16. A control system as claimed in claim 15 wherein said reductionrestraining means comprises a condition discriminating means forcomparing said second operating parameter with a predetermined value,and for holding said second throttle valve in said fully open positionby disabling said first control means from reducing the vehicle drivingtorque with said first actuating means when said second vehicleoperating parameter is greater than said predetermined value.
 17. Acontrol system as claimed in claim 16 wherein said conditiondiscriminating means comprises a partial disabling means which disablessaid first control means from reducing the vehicle driving torque withsaid first actuating means, but enables said second control means toreduce the vehicle driving force with said second actuating when saidsecond operating parameter representing the road surface frictionalcondition is greater than the predetermined value.
 18. A control systemas claimed in claim 16 wherein said condition discriminating meanscomprises a total disabling means for disabling both of torque reductioncontrol operations of said first and second control means when saidsecond parameter representing a driver's accelerator input command isgreater than the predetermined value; and said second vehicle operatingcondition sensed by said second sensing means to determine said secondvehicle operating parameter is one of said accelerator condition whichis indicative of an opening degree of said first throttle valve, and theengine operating condition which is responsive to the acceleratorcondition.
 19. A control system as claimed in claim 15 wherein saidreduction restraining means comprises an adjusting means for increasingan adjustable variable in accordance with said second operatingparameter and a slip discriminating means for producing a slipdiscrimination signal when said first operating parameter is greaterthan said predetermined slip level, but smaller than a predeterminedthreshold level which is higher than said predetermined slip level, andpreventing said first control means from closing said second throttlevalve when said slip discrimination signal is present, said adjustablevariable being one of said threshold level and said second manipulatedquantity of said second actuating means.
 20. A control system as claimedin claim 19 wherein said second control means comprises a manipulatedvariable determining means for reducing the vehicle driving torque byincreasing said second manipulated variable of said second actuatingmeans with increase in a deviation of said first operating parameterfrom a predetermined standard according to at least one of aproportional control action, an integral control action and a derivativecontrol action, and said adjusting means comprises a control gainincreasing means for increasing a control gain which is one of aproportional gain, an integral gain and a derivative gain, used by saidmanipulated variable determining means, in accordance with said secondoperating parameter.
 21. A control system as claimed in claim 15 whereinsaid second sensing means comprises a longitudinal acceleration sensorfor sensing a longitudinal acceleration of the vehicle, and a lateralacceleration sensor for sensing a lateral acceleration of the vehicle;and said controlling means comprises a second parameter determiningmeans for determining said second operating parameter which isproportional to a square root of a sum resulting from addition of asecond power of the longitudinal acceleration and a second power of thelateral acceleration.
 22. A control system as claimed in claim 15wherein said controlling means comprises a second parameter determiningmeans for determining said second operating parameter which isproportional to a product resulting from multiplication of a firstvariable representing an engine output torque by a transmission ratio.23. A control system as claimed in claim 15 wherein said second sensingmeans comprises a first sensor means for sensing a road conditionindicative vehicle operating condition to determine a road conditionindicative vehicle operating parameter and a second sensor means forsensing an accelerator input indicative vehicle operating condition todetermine an accelerator input indicative parameter; and said reductionrestraining means comprises a first restraining means for restraining atorque reduction control operation of said first control means when saidroad condition indicative operating parameter is increased, and a secondretraining means for restraining the torque reduction control operationof said first control means when said accelerator input indicativeoperating parameter is increased.